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Abstract A model for the velocity of proppant particles in slot flow is presented. The proppant is either retarded or accelerated relative to the fluid depending on the ratio of the proppant size to the fracture width. It has been found that when this ratio is small, the proppant travels faster than the average fluid velocity at that location because the proppant tends to be confined to the center of the flow channel where the fluid velocity is higher. As the proppant size increases, the effect of the fracture walls becomes more important and the proppant is retarded by the walls. The retardation of particle relative to the fluid is greater for larger particles and greater proximities to the fracture walls due to the hydrodynamic stress exerted on the sphere by the walls in the narrow gap. A higher proppant concentration restricts the area available to flow and increases the drag forces on the particles. A model is presented for the effect of fracture walls and proppant concentration on proppant transport. The effect of this increased drag force is accounted for by modifying the wall - particle interaction. The influence of the surrounding proppant spheres on the drag force on a particle is estimated from the effect of a wall on the drag force acting on a single particle. The equivalent hydraulic diameter is then used to determine the proppant retardation. The effects of wall roughness and fluid leakoff are discussed. Models are suggested that capture these first order effects. The new model for proppant retardation has been incorporated into a 3D fracture simulator. Results show that the proppant placement is substantially different when proppant retardation/acceleration is considered. Comparisons of propped fracture lengths obtained with the new model agree much better with propped and effective fracture lengths reported in the field. 1.Introduction Hydraulic fracturing is a commonly used stimulation technique. Proppant transport is a key factor in determining the productivity of these fractured wells. Water fracs are common stimulation treatments for low permeability gas reservoirs. These treatments use low viscosity Newtonian fluids to create long narrow fractures in the reservoir, without the excessive height growth that is often seen with cross-linked fluids. The low viscosity fluid and the narrow fractures introduce some significant challenges for proper proppant placement. The low viscosity of the carrying fluid leads to high settling velocities for the proppant. The narrow fractures created can have widths comparable to the diameter of the proppant and can alter proppant transport significantly due to the hydrodynamic forces acting on the proppant because of the fracture walls. Other proppant particles create additional hydrodynamic drag forces leading to retardation. Fracture diagnostic studies that have been reported in the literature have observed that the effective propped lengths for both water fracs and conventional gelled fracs are sometimes significantly different than those predicted by fracture models. Designed and created fracture lengths are usually much longer than the effective fracture lengths obtained from post production analysis[1–4]. They can sometimes be an order of magnitude lower. Proppant transport is a key factor determining the effective propped lengths and therefore the productivity of these fractured wells. In current hydraulic fracture models, the proppant is assumed to flow with the fluid in the direction of fracture propagation. It is shown in this paper that the proppant usually flows at a different velocity than the fluid, particularly in narrow fractures.It is important to develop reliable models to predict proppant transport. A detailed model for proppant settling in water fracs was presented earlier by the authors5. Several correlations for modeling proppant settling in water fracs were presented. These UTFRAC correlations allow fracture models to correct the settling velocity for inertial effects, proppant concentration, fracture width and turbulence. The models were implemented in a 3-D hydraulic fracture simulator and results showed that propped fracture lengths could vary significantly when settling was properly accounted for.
Abstract During hydraulic fracturing, natural fractures and bedding planes can intersect with growing hydraulic fractures and form complex fracture networks. This can result in the flow of fluid and proppant in convoluted fracture pathways with highly variable fracture width and height. Existing models of hydraulic fracturing assume a planar fracture geometry and are unable to simulate proppant placement in such complex networks. In this work, we investigate proppant transport in growing fracture networks using a fully three-dimensional, geomechanical fracture flow, network model with the ability to simulate proppant transport. A three-dimensional hydraulic fracturing simulator developed using the displacement discontinuity method is coupled with a network model for proppant transport. The simulator captures the effect of proppant concentration, fracture width, and fluid rheology on proppant transport. The equations for the fracture network geomechanics, the fluid flow, and the proppant transport are solved in a coupled manner. This provides an accurate estimation of both the fluid pressure and the proppant distribution as the fracture network grows. The geometry of each fracture segment affects the flow distribution in the network. Simulations are then conducted to study the redistribution of proppant as it settles in the fracture network during shut-in to get the final proppant distribution in the network. It is observed that changes in the in-situ stress due to heterogeneity and the stress-shadow induced near the intersection of a hydraulic fracture and a natural fracture may reduce the fracture width and suppress the ability of the proppant to move into the natural fracture. In low permeability formations, due to low leak-off rates, the proppant almost always forms a proppant bank at the bottom of the fracture during shut-in. For planar fractures, proppant settling may disconnect the conductive proppant bank from the wellbore, isolating the productive propped fracture from the wellbore. This problem is exaggerated in the case of fracture networks, where every intersection point between fractures can potentially act as a bottleneck for the flow of produced hydrocarbons. The increase in the surface area due to hydraulically connected natural fractures increases fluid leak-off, reduces the average width of the fracture network, increases proppant concentration, and increases the likelihood of proppant bridging. This work allows us to improve our understanding of proppant placement in three-dimensional, mechanically interacting, complex fracture networks. By coupling geomechanics with proppant transport in fracture networks, it is now possible to study the impact of the stress shadow on proppant placement in natural fractures. The results will assist in improving hydraulic fracture design for naturally fractured reservoirs.
Proppant Transport Behavior in Inclined Versus Vertical Hydraulic Fractures: An Experimental Study
Ba Geri, Mohammed (Missouri University of Science and Technology) | Imqam, Abdulmohsin (Missouri University of Science and Technology) | Dunn-Norman, Shari (Missouri University of Science and Technology)
Abstract Understanding proppant transport in complex fracture systems plays an essential role in determining propped fracture area, fracture conductivity, and their impact on well productivity and economics. Despite extensive, historical work that has studied proppant transport in vertical fractures, very limited investigation exists regarding proppant transport appraisal in inclined hydraulic fractures. This study provides a better understanding of proppant distribution in inclined hydraulic fractures. Proppant transport is governed by several factors such as varying of slurry velocity, fracture geometry, proppant size, and proppant concentration. The main purpose of this experimental study is to evaluate the proppant settling and transport and to determine fracture propped area as a function of the key proppant transport factors in different inclined fracture geometry. Low viscosity fracture fluid (slickwater) was used with different particle sizes: 20/40, 40/70, 100- mesh ceramic proppant. To mimic slurry transport in hydraulic fracturing treatments, a 2 ft. × 2 ft. fracture slot model was constructed with gap of 0.25 in. representing the fracture width. Orientation of the fracture model can be adjusted from vertical to inclined positions. Four injection points perpendicular to the wellbore were used to simulate injection through multiple perforations, in addition to single point injection scenarios. Equilibrium dune height (EDL) is expressed in three regions (near the wellbore, in the center of the fracture, and at the fracture tip) for created fractures. Variations in EDL as a function of the number of perforations that contributed during proppant transport are compared for both vertical and inclined fractures. Experimental results show that both fracture inclination and number of contributing perforations impact EDL and propped fracture area. Inclination of fractures can have significant impact on proppant transport due to the friction or contact force, which comes from the fracture wall. This friction impacts settling velocity of the proppant and impacts the proppant distribution efficiency inside the fracture. Increasing fracture inclination angle increases fracture propped area. Finally, this work observed that number and perforations and their position play an important role in proppant transport, particularly in inclined fractures.
- Research Report > New Finding (1.00)
- Research Report > Experimental Study (0.90)
Abstract Propped hydraulic fracture stimulation is widely used to improve field economics. After treatment some fractures have been found to produce proppant back to surface and this can limit the economic gains from the stimulation. In this paper an analysis of laboratory flowback tests is presented in the form of a 3D stability surface. This enables prediction of when initial flowback occurs for plain proppant. Numerical modeling techniques are introduced that can be used to determine the quantity of produced proppant. For the field application of interest, the calculated proppant volumes compare well with that recovered from the first two fracture jobs. The effectiveness of curable Resin Coated Proppants (RCP) is studied by computer simulation. The approach is the same as that taken when making sand production predictions in clastic reservoirs. Laboratory testing is presented to support the conclusions from the simulation work. P. 381
- Europe > Norway (0.46)
- North America > United States > Mississippi > Marion County (0.24)
- Geology > Geological Subdiscipline > Geomechanics (0.93)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.34)
- Europe > Norway > Norwegian Sea > Halten Terrace > PL 479 > Block 6506/12 > Åsgard Field > Smørbukk Field > Åre Formation (0.99)
- Europe > Norway > Norwegian Sea > Halten Terrace > PL 479 > Block 6506/12 > Åsgard Field > Smørbukk Field > Tofte Formation (0.99)
- Europe > Norway > Norwegian Sea > Halten Terrace > PL 479 > Block 6506/12 > Åsgard Field > Smørbukk Field > Tilje Formation (0.99)
- (2 more...)
ABSTRACT: The residual opening of fluid-driven fractures is conditioned by proppant distribution and has a significant impact on fracture conductivity - a key parameter to determine fluid production rate and well performance. A 2D model follows the evolution of the residual aperture profile and conductivity of fractures partially/fully filled with a proppant pack. The model accommodates the mechanical response of proppant packs in response to closure of arbitrarily rough fractures and the evolution of proppant embedment. The numerical model is validated against existing models and an analytic solution. Proppant may accumulate in a bank at the fracture base during slick water fracturing, and as hydraulic pressure is released, an arched zone forms at the top of the proppant bank as a result of only partial closure of the overlaying unpropped fracture. The width and height of the arched zone decreases as the fluid pressure declines, and is further reduced where low concentrations of proppant fill the fracture or where the formation is highly compressible. This high-conductivity arch represents a preferential flow channel and significantly influences the distribution of fluid transport and overall fracture transmissivity. However, elevated compacting stresses and evolving proppant embedment at the top of the settled proppant bed reduce this aperture and partially diminish the effectiveness of this highly-conductive zone, with time. Contrary to conventional wisdom, simulations suggest that, for a given mass of proppant, uniform distribution throughout the full height of the fracture may not be as effective as a wedge at the fracture base with an open-arch formed above. This arched zone results in a higher overall fracture transmissivity than a uniform proppant distribution. However, this may require further demonstration by production simulations since part of the pay-zone might be disconnected from, or poorly-connected to, the preferential pathway for fluid flow, and this may increase the hydrocarbon diffusion length.
- North America > United States (0.68)
- Europe > Norway > Norwegian Sea (0.24)